Bessemer Process, for the manufacture of steel from pig-iron, was introduced by Sir Henry Bessemer in 1856. Its introduction has almost revolutionised the steel trade, nearly thirty times as much steel being now turned out as was produced prior to its invention, and at about one-fifth the cost per ton. Nevertheless, the finer steels have still to be worked up in other ways, for reasons which are evident when the Bessemer method is explained. The principle is very simple. Pig-iron contains from 2 to 5 per cent. of carbon, besides small quantities of numerous other substances, such as silicon, sulphur, phosphorus, manganese, etc. Steel is essentially a compound of iron with .1 to 1 per cent. of carbon, though several other elements are invariably present in small quantity and considerably affect the nature of steel. Hence if we can properly reduce the quantity of carbon in pig-iron, and also eliminate some of the other ingredients, we shall obtain steel.
This is effected in the Bessemer process by a special method of oxidation. Molten pig-iron is run into a converter lined with ganister, a siliceous reducer. Then air is forced through the liquid metal from below by means of blowing-engines. Ordinary converters contain 8 or 10 tons of metal, and the process lasts 20 to 30 minutes. The progress of the reduction is noted by the appearance of the flames issuing from the converter. If the pig-iron be pure, as with Swedish iron, the process is stopped when the correct carbon percentage is reached. If less pure, it is continued till all the carbon is oxidised, and very nearly all the other ingredients, though practically all the phosphorus and sulphur in the original crude metal still remain. When this condition is reached, a definite amount of carbon and other matter is supplied by introducing a known weight of spiegeleisen, which is a special cast-iron of determinate constitution. In this way a steel may be made with the required percentage of carbon, but with the other ingredients to some, extent beyond control. The metal is condensed subsequently by the steam-hammer and the rolling-mill.